Taxonomic signal in the calls of Turdus hauxwelli and T. fumigatus (Passeriformes: Turdidae) challenges the intergradation hypothesis and refines their distribution

Diagnosing cryptic species has long posed a challenge in ornithology. However, advances in vocal and molecular analyses have significantly enhanced species recognition. In the Amazon, the sister species Turdus hauxwelli and T. fumigatus epitomize this difficulty. Previous studies on plumage suggested the existence of an intergradation zone in southern Central Amazonia and Mato Grosso. However, similarities in their songs have often led to misidentifications that obscure their true distributions. This study sought to evaluate whether acoustic characteristics could refine the geographic boundaries of these species, particularly in Central Amazonia and Mato Grosso, and to reevaluate specimens for potential plumage differences. We analyzed recordings of both species, conducted multivariate analyses of vocal characteristics, and performed morphometric and visual assessments of specimens. Our vocal analyses identified two call types - single-note and multiple-note calls - with each species predominantly using one type as its primary call. These calls allowed clear differentiation between the species. Additionally, our data showed that recordings from Mato Grosso pertain exclusively to T. hauxwelli, contradicting prior reports of T. fumigatus in the region and refuting the previously proposed intergradation zone (based on plumage alone). In contrast, neither morphometric analyses nor plumage patterns effectively distinguished the two species. Some T. hauxwelli specimens from Mato Grosso exhibited plumage characteristics indistinguishable from those of T. fumigatus, likely contributing to earlier misclassifications. Our findings emphasize the taxonomic value of vocalizations in thrushes and highlight the need for integrating diverse datasets to improve our understanding of species identity and distribution.